When you register a domain, you own the whole name-space of that domain:

All possible sub-domains of your domain are yours.
All possible URLs on the domain are yours.
All possible email addresses on the domain are yours.

On your domain, you are also free to choose the linked services such as web, email calendar etc. If you are not satisfied with one provider, you are free to switch to another one at your own convenience.

https://migadu.com/freedom/